Memory devices have been widely used in electronic products to provide high storage speed and low power consumption. For example, resistive random access memory (RRAM) device is one possible candidate for next generation non-volatile memory technology due to simple and complementary metal-oxide semiconductor (CMOS) logic compatible process. Each memory cell in a RRAM device is a metal oxide material sandwiched between top and bottom electrodes. By applying appropriate voltage, the state of each memory cell can be changed from high resistance state (HRS) to low resistance state (LRS) or from LRS to HRS. The above switching mechanism is related to oxygen vacancy migration. The low and high resistance states are utilized to indicate a logical data “1” or “0”, thereby allowing for data storage.